Anchoring elements are used in a variety of dental, medical, and surgical procedures where it is desired to embed a mechanical attachment into living bone. One example of a procedure that uses anchoring elements is dental prosthesis using embedded implants.
Dental implants provide a desirable prosthesis for patients who are missing one or more natural teeth. A dental implant includes an anchoring element that is embedded into the jawbone and a prosthetic tooth that is attached to and supported by the anchoring element. The prosthetic tooth may be attached directly to the anchoring element or an abutment fixture may be attached to the anchoring element and support the prosthetic tooth in turn. An appropriate anchoring element will support bone growth that invades the anchoring element such that the anchoring element becomes integrated with the surrounding bone in a process termed osseointegration.
An anchoring element to support a prosthetic tooth may be embedded in what is termed a two-stage procedure. In the first stage, the anchoring element is embedded into the jawbone and the surgical site is then closed. After a period of months the anchoring element will achieve osseointegration. The site of the anchoring element is then re-opened surgically to allow the attachment of a prosthetic tooth.
More recently techniques have been developed that permit a dental implant to be embedded in a one-stage procedure. In a one-stage procedure, the anchoring element is embedded and a prosthetic tooth is immediately fitted. The immediately fitted prosthetic tooth may be an interim prosthesis that allows the soft tissue to properly heal and maintains the spacing and alignment of adjacent teeth during the period of osseointegration. A permanent prosthetic tooth may be fitted at a later date after at least some osseointegration has occurred without requiring an additional surgical procedure.
To achieve successful osseointegration it is necessary that the anchoring element fit closely into the surrounding bone, generally having a gap of no more than 1.5 millimeters. It is also important that the anchoring element not move relative to the surrounding bone during the period of osseointegration. Where the anchoring element cannot be closely fitted to the surrounding bone, it may be necessary to use bone-grafting materials to fill the space between the anchoring element and the surrounding bone.
Molars are commonly missing teeth. The use of an implant to replace a molar presents some special difficulties. Molars generally have multiple roots. The mandibular molars of the lower jaw generally have two roots. The maxillary molars of the upper jaw generally have three roots. In a fresh extraction site the void left by the molar roots presents a site that can be difficult to fit with an anchoring element. In addition, the bone in the molar region of the jaw generally consists of a thin hard layer of cortical bone surrounding a core of softer spongy cancellous bone. The cancellous bone may provide poor support for the anchoring element prior to osseointegration.
A molar is a relatively large tooth and it is desirable to use an anchoring element having a relatively large diameter to fill the void following an extraction. However, the use of a wide anchoring element may require that a significant amount of bone be removed from the extraction site to accommodate the anchoring element. This may leave only a small amount of cortical bone available to support the anchoring element which may be embedded in predominantly cancellous bone. In particular, the use of a wide anchoring element may require removal of the triangular mound-shaped mass of bone that is found between the roots known as the interradicular bone. Thus, it is difficult to place an anchoring element in a fresh molar extraction site with sufficient stability to allow embedding of a dental implant with a one-stage procedure.
Another circumstance that can present difficulties in placing an anchoring element for a dental implant occurs in cases where it is desired to place a dental prosthesis in a healed extraction site. When a molar is lost, the alveolar ridge that supports the teeth is fairly rapidly resorbed. This causes a loss of height of the jawbone in the area of tooth loss. When an anchoring element is to be embedded in a healed extraction site, it may not be possible to place an anchoring element to a very great depth.
In the lower jaw, the presence of the mandibular alveolar nerve in the lower jaw limits the depth of the anchoring element. In the upper jaw the maxillary sinus limits the depth. These limiting anatomical features may require the use of a short anchoring element, perhaps 5 to 8 millimeters in length. The use of short anchoring elements for such cases has a lower rate of long-term success because of the reduced surface area available for osseointegration.
It would be desirable to have an anchoring element that can be used to place a dental implant in the molar area both for fresh extraction sites and for healed sites.